Available evidence indicates that thirty percent of infants born to HIV-1 infected mothers will be with HIV-1 themselves, and are at great risk for development of morbidity and mortality associated with pediatric AIDS. Attempts at early diagnosis in these infants have been hampered by the fact that the standard serological tests for HIV-1 are uninterpretable when positive in the young infant due to the presence of transplacentally acquired maternal IgG in the infants circulation. At the University of Miami, HIV-1 culture attempts are successful 2/3 of the time on first attempt in young infants, and 95% of the time with repeated culture. This approach has allowed for diagnosis in most children before six months of age. DNA amplification by the polymerase chain reaction or PCR has the potential for matching or exceeding the usefulness of viral culture in infancy, but interpretation of its results has not been validated. The studies outlined in this proposal will first examine the performance of PCR and viral culture side by side in a population of known HIV-1 positive and negative children. We will then conduct a prospective diagnostic study of children born to HIV-1 infected mothers in order to measure the relative diagnostic usefulness of PCR vs. HIV-1 culture for the diagnosis of perinatal HIV-1 infection. In addition, we will take advantage of the longitudinal nature of this HIV-1 transmission study to analyze the generation of genotypic variants of HIV-1 during the course of infection, and will examine the temporal aspects of viral gene expression by in vitro amplification of cDNA's derived from viral RNA'S. These studies may shed light on factors that influence the transmission of HIV-1 from mother to infant, and on factors associated with progression to disease.